Controllable semi-conductor element



United States Patent 3,432,733 'CONTROLLABLE SEMI-CONDUCTOR ELEMENT Edouard Eugster, Windisch, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Filed Apr. 14, 1967, Ser. No. 630,927 Claims priority, application Switzerland, Apr. 22, 1966,

5,894/ 66 US. Cl. 317-235 4 Claims Int. Cl. H01l11/00, /00

ABSTRACT OF THE DISCLOSURE A controllable semi-conductor element comprises a circular disc of semi-conductor material having four stratified zones or layers of different conductor type materials to form an n-p-n-p structure. This structure includes an outer annular zone of n+ type material extending over an outer range which adjoins a first annular part-surface of an end face of a circular zone of p-type material. An inner circular zone of p+ material extending over an inner range adjoins a second part-surface of this end face which is concentric with and spaced radially from the outer annular zone and forms a four zone structure with two p-n transitions. A circular internal zone of the n-type material adjoins the circular zone of p-type material and has a higher average doping in the inner range than in the outer range, and an electrode adjoins the end face to provide a surface contact for the outer annular and inner circular part-surfaces. A second electrode adjoins the opposite end face of another circular zone of p-type material which is located adjacent the circular internal zone of n-type material to complete the four zone structure.

The present invention relates to a controllable semiconductor element with one semiconductor disc having stratified zones of different types for forming an n-p-n-p structure.

Controllable semiconductor elements with n-p-n-pstructure, so-called thyristors, have achieved great significance in their function as inertialess switching elements. The blocking current I which, when exceeded, causes the thyristor to strike, i.e. conduct current, is an important characteristic of the thyristor. The aforementioned blocking current depends substantially on temperature and falls with a rise in temperature. A reduction of the blocking current is also accompanied by a reduction of the breakdown voltage in the forward direction so that normal thyristors lose their blocking characteristics at temperatures of only 110 to 130 C. Moreover, the blocking current is not readily reproducible. In order to avoid this disadvantage, it has been proposed to design thyristors with a higher blocking current but in many cases this necessitates an undesirable increase in the driving power.

It is the object of the invention to provide a controllable semi-conductor element of n-p-n-p structure and increased blocking current but not having the aforementioned disadvantage.

The improved semi-conductor element according to the invention is characterized in that the n-p-n-p structure has an external zone of a first conductor type adjoining a first part-surface of an end face of the semi-conductor disc, that a four layer structure is provided with two p-n transition zones, an external zone of the other, second conductor type adjoining on the one hand on a second part-surface of the end face, and on the other hand on a less doped zone of the second conductor type which, in turn, adjoins between the two aforementioned part-sun Patented Mar. 11, 1969 ice faces onto the end face, the inwardly facing edges of the two-surfaces being equidistant and the internal zone of the first conductor type having a higher average doping in the structure range with two p-n transition zones than in the range of the n-p-n-p structure, and that the electrode adjoining the end face provides a surface contact for both part-surfaces.

The foregoing as well as other objects and advantages inherent in the invention will become more apparent from the following detailed description of one embodiment of the invention and from the accompanying drawings wherein:

FIG. 1 is a view in central vertical section through the improved four-layer semi-conductor structure; and

FIGS. 24 are plots of curves showing anode voltage and blocking current characteristics of four-layer semiconductor structure.

With reference now to the drawings and to FIG. 1 in particular, a circular crystal disc has a stratified zone 1 of n-conductor type onto which the zones 2 and 3 respectively of the p-conductor type adjoin on both sides. An outer annular zone 4 of higher doping and functioning as emitter (n+) and being of the n-type adjoin on a first part-surface of the end face 5 corresponding to the radial range I in which an n-p-n-p structure is accordingly formed. A central circular zone 6 of higher doping (p+) and being of p-conductor type adjoins on the one hand on a circular second part-surface of the end face 5 corresponding to the range II and on the other hand on the zone 2 of the p-conductor type so that a four-layer structure (p+-p-n-p) with two p-n transition zones is formed in the aforementioned range II. The zone 2 adjoins the end face 5 between the two part-surfaces in a range III. It is important that the inwardly facing edges 10, 11 of the part-surfaces of the end face 5 corresponding to the ranges I and II are as equidistant as possible. The zone 1 of the n-conductor type has a higher average doping in the range II than in the range I. This is simply achieved by originating the silicon crystal disc from a cylindrical silicon monocrystal produced by zone refining in an atmosphere containing the doping atoms. By means of this doping method, a doping profile is obtained in the monocrystal having a maximum in the axis and a content of to of its maximum value towards the peripheral zone.

The electrode 7 adjoining the end face 5 and, in the illustrated case, serving as the cathode, is in surface contact with the part-surfaces of the end face 5 correspond ing to the ranges I and II.

The method of operation of the semiconductor element will be described in detail by reference to FIGS. 2 to 4.

FIG. 2 shows the characteristic of the four-layer (p+- p-n-p) structure of the range II while FIG. 3 illustrates the known characteristic of the n-p-n-p structure.

Owing to the higher doping of the zone 2 in range II the avalanche breakdown voltage in that range is smaller than in the range I. If the anode voltage V rises, breakdown will therefore occur at point A, the entire blocking current flowing via the range II and increasing approximately proportionally with a further increase of the anode voltage until it produces a sufiiciently large carrier density on the emitter edge 10 of the n-p-n-p structure in order to cause striking in the range I which then in turn absorbs the entire current.

As can be easily seen, the semi-conductor element characteristic shown in FIG. 4 is obtained as a first approximation from the superposing of the characteristics of the structures in the ranges II or I, respectively according to FIGS. 3 or 2 respectively. The magnitude of the actual striking current however depends on the distance of the ranges II and I respectively, while striking, which is initiated as uniformly as possible over the emitter edge 10, is ensured by maintaining as accurately as possible conditions of equidistance of the emitter edge 10 from the edge 11 of the zone 6.

As can be seen from FIG. 4, the blocking current I is substantially larger than with a normal n-p-n-p structure according to FIG. 3 with the driving power being substantially the same. Since striking is controlled in the first place by the conditions prevailing in the p+-p-np structure in the range II which do not substantially vary over a wide temperature range, the striking current which substantially corresponds to the blocking current I is largely unaffected by temperature changes.

To produce the contacted semi-conductor disc, the zones 2 and 3 are first produced by a diffusion method on a silicon disc of the n-conductor type whose doping is a maximum along the axis. To form the zones 4 or 6 respectively, metal discs 12 or 13, respectively containing the appropriate doping substances are then simultaneously alloyed onto the end face 5 of the semi-conductor disc. The electrodes may then be contacted by any suitable method, always providing that the cathode 7 does not touch the end face 5 of the semi-conductor disc in the range III.

The lowermost zone of p-conductor type terminates in the opposite end face 8 of the four zone semi-conductor structure and is faced with an electrode 9. A terminal connection 10 is applied to the periphery of zone 2 of P' YP I claim:

1. A controllable semi-conductor element comprising a disc of semi-conductor material having four stratified zones of different conductor type materials and forming an n-p-n-p structure, said structure including an outer zone (4) of a first conductor type material extending over an outer range I and which adjoins a first partsurface of an end face (5) of a zone (2) of a second conductor type material, an inner zone (6) of said second conductor type material adjoining on the one hand a second part-surface of said end face and on the other hand a less doped portion of said zone (2) of second conductor type material which extends between said inner and outer zones to said end face thereby to establish over an inner range II a four zone structure with two p-n transitions, the inwardly facing edges (10, 11) of said first and second part-surfaces being uniformly spaced from one another, an internal zone (1) of said first conductor type material adjoining said zone (2) of said second conductor type material and having a higher average doping in said inner range II than in said outer range I, and an electrode (7) adjoining said end face (5) and providing a surface contact for said first and second part-surfaces.

2. A controllable semi-conductor element as defined in claim 1 wherein said first and second conductor types are nand p-materials respectively.

3. A controllable semi-conductor element as defined in claim 1 wherein said disc has a circular configuration, wherein said first-part surface corresponding to range I is constituted by an outer annular edge zone on said end face of said circular disc, and said second part-surface corresponding to range II is constituted by a circular zone on said end face concentric with said annular edge zone and located radially inward thereof and spaced therefrom.

4. A controllable semi-conductor element as defined in claim 1 wherein first and second metal discs (12, 13) having the dimensions respectively of said first and second part surfaces of said end face and containing different doping substances are alloyed onto said zone (2) of said second conductor type material for establishing respectively said outer zone (4) and said inner zone (6).

References Cited UNITED STATES PATENTS 3,277,352 10/1966 Hubner 317--234 3,280,392 10/1966 Benda 317235 3,337,782 8/1967 Todaro 317-235 3,337,783 8/1967 Stehney 317235 3,343,048 9/1967 Kuehn et al 3l7-234 JOHN W. HUCKERT, Primary Examiner.

I. R. SHEWMAKER, Assistant Examiner. 

